Stumped by the Code? Code Rule for Sizing Short Circuit and Ground Fault Protection for a Motor Feeder
Q. What is the Code rule for sizing short circuit and ground fault protection for a motor feeder?
A. Feeder conductors must be protected against short circuits and ground faults by a protective device sized not more than the largest rating of the branch-circuit short-circuit and ground-fault protective device for any motor, plus the sum of the full-load currents of the other motors in the group [430.62(A)].
Let’s run through a specific example problem to highlight how this would be done.
What size feeder protection (inverse time breakers with 75°C terminals) and conductors are required for the following two motors (Figure)?
Motor 1 — 20 hp, 460V, 3-phase, 27A FLC [Table 430.250]
Motor 2 — 10 hp, 460V, 3-phase, 14A FLC
Step 1: Determine the feeder conductor size [430.24]: (27A × 1.25) + 14A = 48A
An 8 AWG conductor is rated 50A at 75°C [110.14(C)(1) and Table 310.15(B)(16)]
Step 2: Feeder protection [430.62(A)] isn’t greater than the largest branch-circuit ground-fault and short-circuit protective device plus the other motor FLC.
Step 3: Determine the largest branch-circuit ground-fault and short-circuit protective device [430.52(C)(1) Ex]:
20-hp motor = 27A × 2.50 = 68A, next size up = 70A
10-hp motor = 14A × 2.50 = 35A
Step 4: Determine the size of the feeder protection:
Not more than 70A + 14A = 84A, next size down = 80A [240.6(A)]
The “next size up protection” rule for branch circuits [430.52(C)(1), Exception No. 1] doesn’t apply to a motor feeder protection device rating.
Q. What are the Code requirements for motor disconnects?
A. A disconnecting means is required for each motor controller, and it must be located within sight from the controller [430.102(A)]. According to Art. 100, within sight means that it’s visible and not positioned more than 50 ft from one to the other.
A motor disconnect must be provided in accordance with Sec. 430.102(B)(1) or (B)(2).
1) Separate motor disconnect. A disconnecting means is required for each motor, and it must be located in sight from the motor location and the driven machinery location.
2) Controller disconnect. The controller disconnecting means [430.102(A)] can serve as the disconnecting means for the motor, if the disconnect is located in sight from the motor location.
There are exceptions to (1) and (2) above. A motor disconnecting means isn’t required under either condition (a) or (b), if the controller disconnecting means [430.102(A)] is lockable, as described in 110.25.
a) If locating the disconnecting means is impracticable or introduces additional or increased hazards to persons or property.
b) In industrial installations, with written safety procedures, where conditions of maintenance and supervision ensure only qualified persons will service the equipment.
Informational Note for Item 2: For information on lockout/tagout procedures, see NFPA 70E, Standard for Electrical Safety in the Workplace.
